This invention relates generally to an electric power system in which a variable amount of electric power is supplied to an electric load circuit from a controllable source that is controlled by regulating means responsive to a given command signal and to a plurality of feedback signals representative, respectively, of various system parameters, and it relates more particularly to improved means for detecting any abnormal loss of feedback signals representative of certain vital system parameters.
The invention is described in the context of a controller for a large self-propelled traction vehicle, such as a locomotive wherein a thermal prime mover (typically a 16-cylinder turbocharged diesel engine) is used to drive an electrical transmission comprising generating means for supplying electric current to a plurality of direct current (d-c) traction motors whose rotors are drivingly coupled through speed-reducing gearing to the respective axle-wheel sets of the vehicle. The generating means typically comprises a main 3-phase traction alternator whose rotor is mechanically coupled to the output shaft of the engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase stator windings of the alternator. These voltages are rectified and applied to the armature windings of the traction motors.
During the "motoring" or propulsion mode of operation, a locomotive diesel engine tends to deliver constant power, depending on throttle setting and ambient conditions, regardless of locomotive speed. Historically, locomotive control systems have been designed so that the operator can select the desired level of traction power, in discrete steps between zero and maximum, and so that the engine develops whatever level of power the traction and auxiliary loads demand.
Engine horsepower is proportional to the product of the angular velocity at which the crankshaft turns and the torque opposing such motion. For the purpose of varying and regulating the amount of available power, it is common practice to equip a locomotive engine with a speed regulating governor which adjusts the quantity of pressurized diesel fuel (i.e., fuel oil) injected into each of the engine cylinders so that the actual speed (RPM) of the crankshaft corresponds to a desired speed. The desired speed is set, within permissible limits, by a manually operated lever or handle of a throttle that can be selectively moved in eight steps or "notches" between a low power position (N1) and a maximum power position (N8). The throttle handle is part of the control consol located in the operator's cab of the locomotive. The position of the throttle handle determines the engine speed setting of the governor.
For each of its eight different speed settings, the engine is capable of developing a corresponding constant amount of horsepower (assuming maximum output torque). When the throttle notch 8 is selected, maximum speed (e.g., 1,050 rpm) and maximum rated gross horsepower (e.g., 4,000) are realized. Under normal conditions the engine power at each notch equals the power demanded by the electric propulsion system which is supplied by the engine-driven main alternator plus power consumed by certain electrically and mechanically driven auxiliary equipments.
The output power (KVA) of the main alternator is proportional to the product of the rms magnitudes of generated voltage and load current. The voltage magnitude varies with the rotational speed of the engine, and it is also a function of the amount of current in the alternator armature and field windings, respectively. For the purpose of accurately controlling and regulating the power supplied to the electric load circuit, it is common practice to adjust the field strength of the traction alternator to compensate for load changes and to minimize the error between actual and desired KVA. The desired power demands on the specific speed setting of the engine. Such excitation control will establish a balanced steady-state condition which results in a substantially constant, optimum electrical power output for each position of the throttle handle.
The alternator output regulating function is performed by an associated controller which is responsive to the throttle position and to a plurality of feedback signals representative, respectively, of certain parameters or quantities (such as the magnitudes of the alternator output voltage and current) of the electric propulsion system. If, due to a fault in the voltage or current sensor or the like, a vital feedabck signal were lost, the regulator would malfunction. In this abnormal event, the regulator would cause the alternator to supply more than the desired KVA to the traction motors, a "wheelslip" condition would develop, and the propulsion equipment could be damaged if the operator fails to move the throttle handle to a lower notch in fast enough response to the loss of wheel adhesion.